Methods and arrangements in a telecommunication system

ABSTRACT

There is provided a method in a cellular telecommunications network, the cellular telecommunications network comprising at least a first cell and a second cell. The method comprises the steps of transmitting first periodic physical signals, usable by a device to determine its location, to the first cell; and transmitting second periodic physical signals, usable by a device to determine its location, to the second cell. The second periodic physical signals are synchronized with the first periodic physical signals and have a timing offset, such that the first periodic physical signals and the second periodic physical signals are not transmitted simultaneously. The method is characterized in that transmission of data or control signals to the first cell is inhibited when the second periodic physical signals are transmitted to the second cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/564,275, filed on Dec. 9, 2014, which is a continuation of U.S.patent application Ser. No. 13/131,378, filed on May 26, 2011, now U.S.Pat. No. 8,938,254, issued on Jan. 20, 2015, which is a 371 ofInternational Application No. PCT/SE2009/050755, filed on Jun. 17, 2009,which is related to, and claims priority from, U.S. Provisional PatentApplication No. 61/120,203, filed on Dec. 5, 2008, the disclosures ofwhich are incorporated here by reference.

FIELD OF THE INVENTION

The present invention relates to methods and arrangements in atelecommunication system, and in particular to a method andcorresponding apparatus allowing detection in a device of signals fromcells that may not be the serving cell of the device.

BACKGROUND

The possibility of determining the position of a mobile device in awireless telecommunication network has enabled application developersand wireless network operators to provide location based, and locationaware, services. Examples of those include guiding systems, shoppingassistance, friend finder, presence services, community andcommunication services and other information services giving the mobileuser information about their surroundings.

In addition to these commercial services, the governments of severalcountries have put requirements on network operators to be able todetermine the position of an emergency call. For instance, thegovernmental requirements in the USA (FCC E911) state that it must bepossible to determine the position of a certain percentage of allemergency calls. The requirements make no distinction between indoor andoutdoor environments.

In outdoor environments, the position estimation can be done usingpositioning systems, e.g. GPS (Global Positioning System) based methodslike Assisted-GPS (A-GPS). Position estimation can also be performedusing the wireless network itself. Methods using the wireless networkcan be arranged into two main groups: those using measurements from asingle radio base station, and those using measurements from a pluralityof radio base stations.

The first group comprises methods that are based on the radio cell towhich a mobile terminal is attached, e.g. by using Cell-ID or acombination of cell-ID and Timing Advance (TA). The TA measurementprinciple is depicted in FIG. 1.

A radio base station 10 serves three radio cells 12 a, 12 b, 12 c.Although three cells are depicted in this example, in general each radiobase station will serve one or more radio cells. In order to determinethe location of a mobile terminal 14, the travel time of radio wavesfrom the radio base station 10 to the mobile terminal 14 and back ismeasured. The distance r from radio base station 10 to mobile terminal14 then follows from the formula:

$r = {c\frac{TA}{2}}$

where TA is the round trip time and where c is the speed of light.

The round trip time measurement alone defines a circle, or if theinaccuracy is accounted for, a circular strip around the radio basestation 10 (more accurately, a sphere, or spherical shell) is defined.By combining this information with the cell polygon, angular extent of apart-circular strip 16 that defines the possible position of the mobileterminal 14 can be computed.

In several systems, therefore, among those Release 8 of the 3GPPspecifications (also known as long term evolution, or LTE), the roundtrip time TA can be used to identify the distance from the antenna atwhich a mobile terminal is positioned. However, it is not possible usingthis method to ascertain where exactly in the sphere or sector the UEis. If TA measurements determine that the mobile terminal is for example500 m from the radio base station, this is along an arc in a sector orcircumference of a circle.

To overcome this problem, a second group of methods uses round trip timemeasurements from a plurality of radio base stations. By determining itsdistance from a plurality of radio base stations, a mobile terminal canmore accurately triangulate its position.

However, modern telecommunications systems are designed to provide highdata rates in the downlink and the uplink (i.e. in communications to andfrom the mobile terminal). It is also desirable to reduce power usage inthe mobile terminal, in order to prolong the battery life as much aspossible. Both of these requirements mandate a high quality radio linkbetween the mobile terminal and its serving radio base station (i.e. theradio base station associated with the mobile terminal's serving radiocell). Thus, interference from other neighbouring radio base stationsshould be kept to a minimum, and in modern telecommunication systemsthis is very successfully achieved. A mobile terminal wishing todetermine its location, however, has difficulty in detecting signalsfrom neighbouring radio base stations for this very reason.

What is required, therefore, is a method whereby a mobile terminal candetect signals from radio base stations other than its serving radiobase station, for example, in order to determine its location.

SUMMARY

According to the present invention there is provided a method in acellular telecommunications network, the cellular telecommunicationsnetwork comprising at least a first cell and a second cell. The methodcomprises the steps of transmitting first periodic physical signals,usable by a device to determine its location, to the first cell; andtransmitting second periodic physical signals, usable by a device todetermine its location, to the second cell. The second periodic physicalsignals are synchronized with the first periodic physical signals andhave a timing offset, such that the first periodic physical signals andthe second periodic physical signals are not transmitted simultaneously.The method is characterized in that transmission of data or controlsignals to the first cell is inhibited when the second periodic physicalsignals are transmitted to the second cell.

The method may be performed by a single radio base station transmittingto the first and second cells, or by a first radio base stationtransmitting to the first cell and by a second base station transmittingto the second cell.

Transmission of data or control signals to the first cell may beinhibited by defining a set of resource elements in which periodicphysical signals are transmitted and in which other data or controlsignals are not. Sets of resource elements may be defined for each suchcell, with the sets of resource elements being substantiallysimultaneous, so that when periodic physical signals are transmitted to,for example, the first cell no other cell in the vicinity of the firstcell is transmitting.

In this way, a mobile terminal in the first or second cell can moreeasily detect the periodic physical signals from each cell even if, forexample, the first cell is its serving cell (or another stronglyinterfering cell). The mobile terminal can then take timing measurementson the periodic physical signals and so determine its location.

A cellular telecommunication system and a radio base station forperforming the above method are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the following drawings, in which:

FIG. 1 illustrates a method of determining the position of a terminal ina telecommunications network;

FIG. 2 illustrates a method of determining the position of a terminal ina telecommunications network according to embodiments of the presentinvention;

FIG. 3 illustrates a radio base station according to embodiments of thepresent invention;

FIG. 4 illustrates a telecommunication system according to an embodimentof the present invention;

FIG. 5 illustrates a signalling arrangement for a radio base station ofthe system of FIG. 4;

FIG. 6 illustrates a telecommunication system according to anotherembodiment of the present invention;

FIG. 7 illustrates a signalling arrangement for the system of FIG. 6;

FIG. 8 is a flowchart of a method according to embodiments of thepresent invention; and

FIG. 9 shows synchronization signals according to embodiments of thepresent invention.

DETAILED DESCRIPTION

FIG. 2 shows a telecommunication system 20.

The telecommunication system 20 may be, for example, an evolveduniversal terrestrial radio access network (E-UTRAN) for use withRelease 8 or any later release of the 3GPP specifications, or any otherwireless telecommunication network such as CDMA2000, GSM, WLAN, etc.

The system 20 comprises a terminal 22, which in the illustrated exampleis a mobile terminal, also called a user equipment or mobile station.The invention is also applicable to stationary terminals.

The system 20 further comprises a plurality of radio base stations 24,26, 28, of which three are shown here. One of the radio base stations,referenced 24, is the serving radio base station, which maintains theparticular radio cell with which the terminal 22 is registered, as willbe appreciated by those skilled in the art. Moreover, each radio basestation 24, 26, 28 may maintain more than one radio cell. In operation,therefore, the terminal 22 primarily sends transmissions to (uplink),and receives transmissions from (downlink), the serving cell, which ismaintained by the serving radio base station 24.

The terminal 22 may also be able to detect signals from neighbouringradio base stations 26, 28 or signals from cells that are not theserving cell; however, these signals will in general be much weaker thanthose from the serving radio base station 24.

As previously mentioned, at certain instances it is useful to determinethe geographical location of the terminal 22. This may be instigated bythe terminal 22 itself, or by the network, for example if the terminal22 is making an emergency call. In the latter case, the terminal 22receives an instruction from the serving radio base station 24 todetermine its location.

When determining its location, in one embodiment the terminal 22 takestime of arrival (TOA) measurements from each of the radio base stations24, 26, 28 in its vicinity. The measurements allow the terminal 22 todetermine a measure of the distance (in practice, a pseudo-distance)from each radio base station, in effect generating circles (or circularstrips, taking into account inaccuracy in the measurements) whose radiiis equal to the distance, or pseudo-distance, of the terminal 22 fromeach respective radio base station. The use of “pseudo-distance” arisesbecause of the receiver clock bias in the terminal 22 (see equations(1a) to (1n) below). In such an embodiment, the absolute distance fromeach radio base station is not measured. The terminal 22 can thendetermine its location as being at the intersection of these circles.

The TOA principle can be written in mathematical form as follows.

t _(R) ¹ =t _(T) ¹+√{square root over ((x−x ₁)²+(y−y ₁)²)}{square rootover ((x−x ₁)²+(y−y ₁)²)}/c+b+ν ¹  (1a)

t _(R) ² =t _(T) ²+√{square root over ((x−x ₂)²+(y−y ₂)²)}{square rootover ((x−x ₂)²+(y−y ₂)²)}/c+b+ν ²  (1b)

. . .

t _(R) ^(N) =t _(T) ^(N)+√{square root over ((x−x _(N))²+(y−y_(N))²)}{square root over ((x−x _(N))²+(y−y _(N))²)}/c+b+ν ^(N)  (1c)

where:t_(R) ^(i): Time of reception by the terminal for the ith base station(measured)t_(T) ^(i): Time of transmission from the ith base stationx_(i), y_(i): Coordinates of ith base stationc: Speed of lightx, y: Coordinates of MS computed by solving equations (at MS or innetwork node)b: receiver clock biasv^(i): Measurement error of ith timing measurementt_(T) ^(i) may be provided to the terminal 22 in a number of ways. Forexample, t_(T) ^(i) may be provided with assistance data, or known tothe terminal 22 in a synchronized network. The coordinates of the ithbase station, x_(i) and y_(i), are known in the network, and may betransmitted to the terminal 22, or the terminal 22 could maintain alocal database of base station coordinates.

The equations (1a) to (1n) can be solved for the unknowns (x, y, b)whenever n≧3 and the geometry of the base stations is good, i.e. spacedsuch that the terminal 22 has to look in a different direction for eachbase station. One method of solving the equations is to use numericaloptimization solutions based on Taylor series expansions of equations(1a) to (1n), although alternative methods are well known in the art.

The equations (1a) to (1n) may be solved in the terminal 22 itself, orremotely in the network, in which case the terminal 22 transmits thetiming measurements to the network via the serving base station 24.

In addition to the TOA-based method described above, alternative methodsof positioning will be known to those skilled in the art. For example,time difference of arrival (TDOA) methods measure the difference inarrival times at different base stations of a pulse signal transmittedby the terminal 22.

Positioning using a timing-based method therefore requires that thetiming of at least three geographically dispersed radio base stations ismeasured. It is necessary to ensure that the signal-to-noise ratio (SNR)to the third strongest base station is strong enough so that it canstill be detected by the terminal 22. Cellular systems which reuse thesame frequency band are designed to create strong isolation betweencells, meaning that the signal from the own serving cell should bestrong while interference from the neighbouring base stations should beminimized. This means that the requirements for positioning andcommunication are conflicting. Since modern telecommunication systemsare primarily for communication, time measurements for positioning needto be done at very low C/I (carrier to interference ratio) toneighbouring base stations, which puts high requirements on the terminalreceiver and also typically degrades the positioning accuracy. Forexample, in a setup that minimizes inter-cell interference the C/I tothe third base station may be very low, −23 dB at the 5% level for thethird strongest base station.

In other circumstances, transmissions from a neighbouring cell may bestrongest and prevent the terminal 22 from detecting signals from athird base station, or even from the serving radio base station 24.

According to embodiments of the present invention, this problem isovercome by offsetting the transmission of periodic physical signals incells that may interfere with one another. For example, a single radiobase station may transmit periodic physical signals to multiple cells,where the periodic physical signals for each cell are offset in timewith respect to each other. Alternatively, in a synchronized system, theoffset transmissions to the cells may come from different radio basestations.

Further, when periodic physical signals are being transmitted in a firstcell but not in a second cell, other transmissions are inhibited in thesecond cell. In this way, interference between cells is minimized, sothat the terminal 22 can more easily detect physical signals fromneighbouring cells as well as its serving cell. This makes it easier,for example, to take the necessary measurements to determine itslocation.

FIG. 3 illustrates a radio base station 30 according to embodiments ofthe present invention. It will be apparent to those skilled in the artthat the radio base station is suitable for use in any cellulartelecommunication network, under any current or future standard.Therefore, the radio base station 30 may also be termed a NodeB, or aneNodeB, for example.

The radio base station 30 comprises a plurality of antennas, 32 ₁ to 32_(N) (where N is an integer greater than one), with at least one of theplurality of antennas 32 responsible for transmitting signals to andreceiving signals from each of the cells the radio base station 30 isdesigned to serve. It will be apparent to those skilled in the art thatmore than one antenna 32 may be used for each cell, resulting inso-called multiple-input, multiple-output (MIMO) communications.

Each antenna 32 is coupled to Rx/Tx circuitry, 34 ₁ to 34 _(N), whichfilters, downconverts to the baseband, and samples signals received bythe antennas 32, or the inverse operations for signals to be transmittedby the antennas 32. It will be apparent to those skilled in the artthat, although the illustrated embodiment depicts individual Rx/Txcircuitry 34 for each antenna 32, one or more Rx/Tx circuitry 34 may becoupled to all of the antennas 32.

Each Rx/Tx circuitry 34 is coupled to processing circuitry 36, whichgenerates and modulates signals to be transmitted by the antennas 32, ordemodulates and interprets signals received by the antennas 32.

As will be familiar to those skilled in the art, the processingcircuitry 36 is further coupled to a memory 38, and an interface 40 to acore network (CN) of the telecommunication system. Numerous otherfeatures usually found in radio base stations have been omitted forclarity.

FIG. 4 shows a telecommunication system 100 according to one embodimentof the present invention.

The telecommunication system 100 comprises three radio base stations102, 104, 106, and each radio base station 102, 104, 106 transmits tothree cells. However, it will be apparent to those skilled in the artthat this embodiment of the present invention contemplates atelecommunication system 100 having two or more radio base stations, andeach radio base station may transmit to two or more cells.

In order to allow terminals within the cells to maintain a connectionwith a respective radio base station 102, 104, 106, each radio basestation transmits periodic physical signals to each of its cells.Moreover, as previously stated, the transmissions of the radio basestations 102, 104, 106 are generally synchronized. These physicalsignals may comprise one or more of reference signals, synchronizationsignals, or dedicated positioning reference signals. Reference signalsare generally transmitted in every subframe; synchronization signals aretransmitted in every fifth subframe.

According to this embodiment of the present invention, thetelecommunication system 100 is synchronized in that each of the radiobase stations 102, 104, 106 transmits periodic physical signals havingsubstantially the same time base (i.e. with the same period). However,the radio base station 102 transmits periodic physical signals to eachof its cells with a different timing offset. For example, in theillustrated embodiment, where the radio base station 102 transmits tothree cells, the radio base station transmits periodic physical signalsto a first cell at time t₀, to a second cell at time t₁, and to a thirdcell at time t₂. In this way, a terminal in the vicinity of radio basestation 102 can more easily detect the periodic physical signals fromeach different cell, as they are transmitted at different times.

In the illustrated embodiment, each of the three radio base stations102, 104, 106 transmits periodic physical signals to each of theirrespective cells using the same timing scheme. Thus, the radio basestation 104 transmits periodic physical signals to its first cell attime t₀, to its second cell at time t₁, and to its third cell at timet₂; and the radio base station 106 transmits periodic physical signalsto its first cell at time t₀, to its second cell at time t₁, and to itsthird cell at time t₂. With a suitable network geometry, therefore, aterminal in between each of the three radio base stations 102, 104, 106will be closest to a cell of each of the radio base stations having adifferent timing offset. Again, in this way, the terminal can moreeasily detect the periodic physical signals from each different radiobase station, as they are transmitted at different times, and so usethose signals to determine its location in accordance with the methodshown in FIG. 2.

In order to further mitigate interference between the cells of differentradio base stations, the signals transmitted by one radio base stationmay be transmitted on a different frequency or set of frequencies, orwith a different scrambling code to those transmitted by other radiobase stations.

Further, according to embodiments of the invention, the transmission ofsignals to other cells when periodic physical signals are beingtransmitted to a first cell is inhibited. For example, when radio basestation 102 transmits periodic physical signals to its first cell attime t₀, transmission of data or physical signals to its second andthird cells is inhibited; likewise, when radio base station 102transmits periodic physical signals to its second cell at time t₁,transmission of data or physical signals to its first and third cells isinhibited; and when radio base station 102 transmits periodic physicalsignals to its third cell at time t₂, transmission of data or controlsignals to its first and second cells is inhibited. The scheme maysimilarly be applied to the other radio base stations 104, 106.

FIG. 5 illustrates an exemplary signalling arrangement for the radiobase station 102, where time, t, is represented by the horizontal axis,and frequency, f, is represented by the vertical axis. Each highlightedblock represents one resource element on which the radio base stationmay transmit, and one subframe is shown for each of the three cellsserved by the radio base station 102.

It will be noted that each of the subframes is offset by the time takento transmit using one resource element, also referred to herein as atimeslot. However, different offsets are contemplated by the presentinvention. Generally, as noted above, the radio base station 102transmits periodic physical signals to cell 0 at time t₀, to cell 1 attime t₁, and to cell 2 at time t₂.

The transmissions to each cell generally take the same format in thisembodiment, with the first two OFDM symbols (i.e. slots in time)reserved for control signals A (medium grey squares). After a furthertwo OFDM symbols, periodic physical signals are transmitted to each cellon two different frequencies B (black squares). This is repeated threeOFDM symbols later, using a different pair of frequencies, and repeatedagain using the original pair of frequencies three OFDM symbols afterthat. Because the subframes for each cell are offset in time withrespect to each other, the periodic physical signals are not transmittedat the same time.

Additionally, the signalling scheme includes resource elements C onwhich transmission is inhibited (light grey). These are resourceelements which correspond in time and frequency to the resource elementson which periodic physical signals are being transmitted to other cells.Thus, there are defined sets of consecutive resource elements (i.e.resource elements that are consecutive in the time domain) in whichtransmission of periodic physical signals occurs, and in which othertransmissions are inhibited. In the first set, for cell 0, periodicphysical signals are transmitted in the first resource element of theset; in the second set, for cell 1, periodic physical signals aretransmitted in the second resource element of the set; and in the thirdset, for cell 2, periodic physical signals are transmitted in the thirdresource element of the set. Each set of resource elements occurs atsubstantially the same time across each of the cells, because cellseither transmit periodic physical signals or are inhibited from doing soduring the time covered by the sets of consecutive resource elements.This allows terminals to detect the periodic physical signals from eachrespective cell more easily.

In the illustrated embodiment, inhibition of other transmissions occursonly in a defined subset of the resource elements in the subframe.However, in some embodiments inhibition of other transmissions occurs inall of the resource elements in the subframe. It will be apparent tothose skilled in the art that not all subframes will have periodicphysical signals transmitted within them.

As will be appreciated by those skilled in the art, radio base stations104, 106 may also transmit periodic physical signals to their respectivecells using the signalling arrangement described above. Therefore, aterminal in one of the central cells of the system described withrespect to FIG. 4 (i.e. cell 0 of radio base station 102, cell 1 ofradio base station 104, and cell 2 of radio base station 106), can moreeasily detect periodic physical signals from each radio base station102, 104, 106, and so may determine its location.

FIG. 6 shows a telecommunication system 200 according to anotherembodiment of the present invention.

Again, the telecommunication system 200 comprises three radio basestations 202, 204, 206, and each radio base station 202, 204, 206transmits to three cells. However, it will be apparent to those skilledin the art that embodiments of the present invention contemplate atelecommunication system 200 having two or more radio base stations, andeach radio base station may transmit to one or more cells.

According to this embodiment of the present invention, the transmissionby a radio base station of periodic physical signals to each of itscells is synchronized. That is, radio base station 202 transmitsperiodic physical signals to each of its three cells at the same time.To avoid interference occurring between these transmissions, the signalsto each cell may be transmitted on a different frequency or set offrequencies, or using a different scrambling code. Likewise, the radiobase station 204 transmits periodic physical signals to each of itsthree cells at the same time, and the radio base station 206 transmitsperiodic physical signals to each of its three cells at the same time.

However, the transmissions of each radio base station 202, 204, 206 areoffset in time with respect to each other. Thus, although they aretransmitted with substantially the same time base (i.e. with the sameperiod), the transmission of the periodic physical signals of each radiobase station does not occur at the same time. For example, the radiobase station 202 transmits periodic physical signals to each of itscells at time t₀, the radio base station 204 transmits periodic physicalsignals to each of its cells at time t₁, and the radio base station 206transmits periodic physical signals to each of its cells at time t₂.

Moreover, according to embodiments of the present invention,transmission by one radio base station of other signals, i.e. controland data signals, is inhibited while other radio base stations aretransmitting periodic physical signals. Thus, when radio base station202 transmits periodic physical signals to each of its cells at time t₀,radio base stations 204, 206 are inhibited from transmitting; likewise,when radio base station 204 transmits periodic physical signals to eachof its cells at time t₁, radio base stations 202, 206 are inhibited fromtransmitting; and when radio base station 206 transmits periodicphysical signals to each of its cells at time t₂, radio base stations202, 204 are inhibited from transmitting.

FIG. 7 illustrates an exemplary signalling arrangement for the radiobase stations 202, 204, 206 where time, t, is represented by thehorizontal axis, and frequency, f, is represented by the vertical axis.Each highlighted block represents one resource element on which theradio base stations may transmit, and one subframe is shown for each ofthe three cells served by each radio base station 202, 204, 206. It isto be further noted that the time axis restarts for each column; so, forexample, radio base station 202 transmits to each of its cells at thesame time, even though they are displaced along the horizontal axis. Forclarity, detailed signalling arrangements are only shown for cell 0 ofradio base station 202, cell 1 of radio base station 204, and cell 2 ofradio base station 206, hereinafter termed the “central” cells in FIG.6.

Again, the first two OFDM symbols of each subframe are reserved forcontrol signalling A (medium grey). For each radio base station 202,204, 206, after a further two OFDM symbols periodic physical signals Bare transmitted (black squares). This occurs at the same time for eachcell served by a particular radio base station, but there is a timingoffset between the transmission of periodic physical signals bydifferent radio base stations. In the illustrated example, periodicphysical signals are transmitted to each cell of a particular radio basestation using a different set of frequencies (i.e. offset vertically).

As can be seen from FIG. 6, the three central cells are in danger ofinterfering with one another, even taking into account the timing offsetbetween them. Therefore, according to embodiments of the presentinvention, the radio base stations are prevented from transmitting onresource elements when periodic physical signals are transmitted onpotentially interfering cells. The detailed signalling arrangement ofcell 0 of radio base station 202 therefore includes a number of limitedtransmission slots C (light grey), on which transmission is inhibited,or prevented altogether. These are the resource elements that correspondto those used by cell 1 of radio base station 204, and cell 2 of radiobase station 206 to transmit periodic physical signals. Similar limitedtransmission slots are defined for these other cells as well to avoidtransmission when radio base station 202 is transmitting periodicphysical signals to its cell 0. As there is a frequency offset betweencells of each radio base station, in this embodiment the limitedtransmission slots are also necessarily offset in frequency.

Therefore this are again defined sets of consecutive resource elements(i.e. resource elements that are consecutive in the time domain) inwhich transmission of periodic physical signals occurs, and in whichother transmissions are inhibited. In the first set, for cell 0 of radiobase station 202, periodic physical signals are transmitted in the firstresource element of the set; in the second set, for cell 1 of radio basestation 204, periodic physical signals are transmitted in the secondresource element of the set; and in the third set, for cell 2 of radiobase station 206, periodic physical signals are transmitted in the thirdresource element of the set. Each set of resource elements occurs atsubstantially the same time across each of the cells, because cellseither transmit periodic physical signals or are inhibited from doing soduring the time covered by the sets of consecutive resource elements.This allows terminals to detect the periodic physical signals from eachrespective cell more easily.

As with earlier embodiments, in the illustrated embodiment inhibition ofother transmissions occurs only in a defined subset of the resourceelements in the subframe. However, in some embodiments inhibition ofother transmissions occurs in all of the resource elements in thesubframe. It will be apparent to those skilled in the art that not allsubframes will have periodic physical signals transmitted within them.

In this way, a terminal positioned in one of the central cells of FIG. 6may more easily detect signals from each of the central cells. Forexample, this may be used as part of a positioning attempt, as describedwith respect to FIG. 2.

Such limited transmission resource elements may also be defined for theother cells of each radio base station; however, for clarity only thosefor three cells are shown.

FIG. 8 is a flowchart of a method according to embodiments of thepresent invention. The method may be performed in a telecommunicationsystem as a whole (i.e. employing more than one radio base station), orin an individual radio base station.

The method begins in step 300.

In step 302, periodic physical signals are transmitted to a first cellof the telecommunication system. The periodic physical signals maycomprise one or more of reference signals, synchronization signals anddedicated positioning reference signals.

In step 304, at a later time (i.e. with a predetermined timing offset),periodic physical signals are transmitted to a second cell of thetelecommunication system.

In an embodiment where the method is performed by a single radio basestation, for example the radio base station 30 described with respect toFIG. 3, the radio base station serves both the first and second cells.That is, the radio base station 30 generates periodic physical signalsusing the processing circuitry 36, and transmits them to the cells usingthe respective antennas 32.

In an embodiment where the method is performed in two or more radio basestations, the periodic physical signals are transmitted to the firstcell by a first radio base station, and to the second cell by a secondradio base station.

In one alternative embodiment of the present invention (illustrated bythe dashed line in FIG. 8), the method proceeds next to step 306, wheretransmissions to the first cell are inhibited at the time that periodicphysical signals are transmitted to the second cell. The inhibition maybe achieved as described with respect to FIGS. 5 and 7, by definingresource elements or OFDM symbols in which transmission is limited, orprevented altogether. After this, the method returns to step 302, andperiodic physical signals are transmitted to the first cell again. Ofcourse, it will be clear that the periodic physical signals will ingeneral not be the immediate next transmission; it is likely thatcontrol and/or data will be transmitted before the periodic physicalsignals are next transmitted.

Alternatively, the method may proceed to step 308, where it isdetermined if an attempt is being made to locate a terminal in thevicinity of the first or second cell. The indication of an attempt tolocate a terminal may be received from the terminal itself, or from acore network of the telecommunication system (for example, if theterminal is making an emergency call).

If a location attempt is begin made, the method proceeds to step 306,where transmissions to the first cell are inhibited at the time thatperiodic physical signals are transmitted to the second cell. Theinhibition may be achieved as described with respect to FIGS. 5 and 7,by defining resource elements or OFDM symbols in which transmission islimited, or prevented altogether. After this, the method returns to step302, and periodic physical signals are transmitted to the first cellagain. Of course, it will be clear that the periodic physical signalswill in general not be the immediate next transmission; it is likelythat control and/or data will be transmitted before the periodicphysical signals are next transmitted.

If a location attempt is not being made in this embodiment, noinhibition of transmission on the first cell occurs. That is, the radiobase station is free to transmit to the first cell when periodicphysical signals are transmitted to the second cell. In this embodiment,therefore, inhibition of transmission occurs only if an attempt is beingmade to locate a terminal in the first or second cells.

As described above, in embodiments of the present invention, theperiodic physical signals may comprise one or more of reference signals,synchronization signals and dedicated positioning reference signals.

In Release 8 of the 3GPP specifications, synchronization signals aretransmitted in subframes 0 and 5 as illustrated in FIG. 9. The primarysynchronization signal (PSS) is transmitted in the last OFDM symbol andthe secondary synchronization signal (SSS) in the penultimate OFDMsymbol of a subframe. There are three different PSS sequences and 168different SSS sequences. The sequence identities are used to distinguishdifferent cells. The identity of the cell then can be used to determinethe reference signal sequence and its allocation in the time-frequencygrid. The synchronization signals occupy 62 resource elements in thecentre of the allocated bandwidth.

Also in Release 8 of the 3GPP specifications, reference symbols aretransmitted on certain resource elements in every subframe and over theentire bandwidth. They are therefore very suitable for use in channelestimation, especially when the timing measurements are to be performedon other signals, e.g. dedicated positioning reference signals that maybe provided in later releases. The dedicated positioning referencesignals could themselves be used for channel estimation.

The present invention therefore provides a convenient method forensuring that a device can detect signals from cells that are notnecessarily its serving cell. This has particular utility, for example,in processes to determine the position of the device, where signals froma plurality of radio base stations are required.

The invention has been described mainly with reference to Release 8 andlater releases of the 3GPP specifications, and OFDM (orthogonalfrequency-division multiplexing) technology; however, it will beunderstood by those skilled in the art that the invention is alsoapplicable to any wireless cellular telecommunications system, and anytransmission technology.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single processor orother unit may fulfil the functions of several units recited in theclaims. Any reference signs in the claims shall not be construed so asto limit their scope.

1. A method in a cellular telecommunications network, the cellulartelecommunications network comprising at least a first cell and a secondcell, the method comprising: transmitting first periodic physicalsignals, usable by a device to determine its location, to the firstcell; and transmitting second periodic physical signals, usable by adevice to determine its location, to the second cell, said secondperiodic physical signals being synchronized with said first periodicphysical signals and having a timing offset, such that said firstperiodic physical signals and said second periodic physical signals arenot transmitted simultaneously, characterized in that transmission ofdata or control signals to the first cell is inhibited when said secondperiodic physical signals are transmitted to the second cell.
 2. Amethod as claimed in claim 1, wherein said transmission of firstperiodic physical signals occurs within a first set of resourceelements, and wherein said transmission of second periodic physicalsignals occurs within a second set of resource elements, the first andsecond sets being substantially simultaneous.
 3. A method as claimed inclaim 2, wherein transmission of data or control signals is inhibitedfor the resource elements of said first and second sets in which saidfirst and second periodic physical signals are not transmitted.
 4. Amethod as claimed in claim 1, wherein said first and second periodicphysical signals are transmitted by a single radio base station.
 5. Amethod as claimed in claim 1, wherein said first periodic physicalsignals are transmitted by a first radio base station and said secondperiodic physical signals are transmitted by a second radio basestation.
 6. A method as claimed in claim 1, further comprising:receiving an indication that an attempt is being made to locate aterminal in an area including said first cell and said second cell;wherein said inhibiting of data or control signal transmission occurs inresponse to said indication.
 7. A method as claimed in claim 6, whereinsaid indication is received from said terminal.
 8. A method as claimedin claim 6, wherein said indication is received from a core network ofthe cellular telecommunications network.
 9. A method as claimed in claim1, wherein said periodic physical signals comprise one or more of:reference signals, synchronization signals and dedicated positioningreference signals.
 10. A cellular telecommunication system, comprising:a first radio base station, for transmitting first periodic physicalsignals, usable by a device to determine its location, to a first cell;and a second radio base station, for transmitting second periodicphysical signals, usable by a device to determine its location, to asecond cell, said second periodic physical signals being synchronizedwith said first periodic physical signals and having a timing offset,such that said first periodic physical signals and said second periodicphysical signals are not transmitted simultaneously, characterized inthat transmission of data or control signals by the first radio basestation to the first cell is inhibited when said second periodicphysical signals are transmitted to the second cell.
 11. Atelecommunication system as claimed in claim 10, wherein saidtransmission of first periodic physical signals occurs within a firstset of resource elements, and wherein said transmission of secondperiodic physical signals occurs within a second set of resourceelements, the first and second sets being substantially simultaneous.12. A telecommunication system as claimed in claim 11, whereintransmission of data or control signals is inhibited for the resourceelements of said first and second sets in which said first and secondperiodic physical signals are not transmitted.
 13. A radio base stationfor a cellular telecommunication network, the radio base station servingat least a first cell and a second cell, the radio base stationcomprising at least one antenna, the antenna being configured totransmit first periodic physical signals, usable by a device todetermine its location, to the first cell; and to transmit secondperiodic physical signals, usable by a device to determine its location,to the second cell, said second periodic physical signals beingsynchronized with said first periodic physical signals and having atiming offset, such that said first periodic physical signals and saidsecond periodic physical signals are not transmitted simultaneously,characterized in that transmission of data or control signals by theantenna to the first cell is inhibited when said second periodicphysical signals are transmitted to the second cell.
 14. A radio basestation as claimed in claim 13, wherein said transmission of firstperiodic physical signals occurs within a first set of resourceelements, and wherein said transmission of second periodic physicalsignals occurs within a second set of resource elements, the first andsecond sets being substantially simultaneous.
 15. A radio base stationas claimed in claim 14, wherein transmission of data or control signalsis inhibited for the resource elements of said first and second sets inwhich said first and second periodic physical signals are nottransmitted.